As is known, co-ordinate measuring machines generally comprise a machine bed designed to support the piece to be measured and a mobile unit for moving a measuring sensor with respect to the machine bed.
More in particular, the mobile unit generally comprises a first carriage that is mobile on the machine bed along guides parallel to a first axis, a second carriage carried by the first carriage and mobile along a second axis orthogonal to the first axis, and a third carriage carried by the second carriage and mobile with respect to this along a third axis orthogonal to the first two axes. The measuring sensor is carried by the third carriage.
In machines of the type briefly described above, the machine bed is normally made of granite, and has the dual purpose of supporting the piece and of defining the guides for the first carriage.
This entails different drawbacks.
In the first place, the positioning of the piece on the machine bed, particularly in the case where the weight of the piece is considerable, leads to a deformation of the machine bed itself, which causes a deformation of the guides and thus induces errors of measurement.
In addition, the movement of the carriages of the mobile unit along the guides, and in particular of the main carriage, induces deformations on the machine bed and thus alters the position of the piece. This thus determines further errors of measurement.
Other drawbacks connected with the use of machine beds made of granite are represented by the cost, the weight, and the difficulty of supply of the granite in a short time.
To solve at least partially the problems linked to the weight of the piece, solutions have been proposed in which the granite bed is uncoupled from the supporting structure.
Illustrated in WO 89/03505 is a measuring machine comprising a metal base carrying the guides for the mobile unit and resting on which is a worktable made of granite.
Illustrated in the document No. GB-A-2080954 is a measuring machine in which a worktable made of hard material is constrained to an underlying metal base provided with guides for the mobile unit via positioning elements without play and such as not to transmit stresses.
Both of the solutions described above of a machine bed require complex and costly base structures and, in any case, use a granite table, with all the drawbacks that this entails.